The long-term objective of the proposed research is to describe and explain changes in auditory behavior that occur between infancy and adulthood in humans. The results of this research will help to define the course of normal auditory development. such information is a prerequisite to understanding abnormal development, as well as the function of the mature system. In addition, the methods developed in the course of this research may prove useful in assessing hearing loss in human infants. The research has five specific aims. The first is to examine the development of frequency resolution, the ability to respond selectively to one component of a complex sound. Previous work has shown that this ability is still immature at 3 months postnatal age. An experiment is proposed to examine frequency resolution in younger infants to follow its development. The second specific aim is to examine the development of temporal resolution, the ability to follow changes in a sound over time. Previous work has shown that infants respond poorly to temporal changes. The proposed work is aimed at determining whether the poor behavioral response is a result of immaturity of the primary auditory system. The third aim is to understand why infants have difficulty discriminating between frequencies under some conditions. Three experiments are proposed to examine frequency discrimination in 2- month-old infants and to determine whether infants' frequency discrimination varies with stimulus conditions as adults' frequency discrimination does. The fourth specific aim is to develop more reliable and sensitive measures of infant auditory behavior. The proposed experiments will attempt to model infant test methods to better understand how methods influence our measures of infant hearing, develop an improved "video reinforcer" for testing infants' hearing, and formalize a method for testing 2- to 4-week-old infants. the final specific aim is to ask how the ability to attend selectively to sound affects the infant's behavioral response. One experiment will determine how the infant's response to sound changes with intensity; differences between infants and adults in this measure are predicted if infants are inattentive to the sound. Two other experiments will address how the infant responds to an unexpected sound, and whether giving the infant additional cues about the timing and the acoustic frequency of a sound will improve detection.